More specifically, this type of cell consists of two electrodes (anode and cathode, electron conductors) which are connected to a direct current generator, and separated by an electrolyte (ion-conducting medium), which may advantageously comprise a proton exchange polymer membrane that makes it possible to not resort to a liquid electrolyte, to have a great compactness, to limit corrosion problems, and to result in substantially better performance.
Oxygen is produced at the anode by oxidation of water (E0=1.23 V/SHE) and hydrogen is produced at the cathode by reduction of the proton (E0=0 V/SHE). The anode materials must withstand high potentials (typically greater than 1.5 V/SHE). Noble metals such as platinum at the cathode or oxides of noble metals (iridium, ruthenium, or alloys of these metals, etc.) at the anode are generally used as electrocatalysts.
A supply of energy to the system enables the anode and cathode reactions and the production of the gases to take place.
The electrolyzer core, also referred to as a membrane electrode assembly (MEA), comprises the proton exchange membrane 20, and the two layers of electrocatalysts, as shown by the references 10 and 30 respectively at the cathode 11 and anode 31.
The objective of a PEM water electrolyzer is to have the highest possible energy efficiency. Specifically, the objective is to be able to produce the desired amounts of gas while reducing the energy consumption (generally expressed in kWh·Nm−3). This is expressed by obtaining the lowest possible electrolysis voltage for a given current.
The components used in the electrocatalytic layers must therefore catalyze the reactions of reduction of the proton (formation of hydrogen) and of oxidation of water (formation of oxygen) that take place respectively at the cathode and at the anode.
Several problems are then identified:
the materials used must be good electron conductors in order to limit the resistance of the system (composed of the ohmic resistances and of the interfacial resistances);
these materials must be stable under the electrolysis conditions (acid medium, stability with respect to the potentials);
since the catalytic materials are generally noble and precious metals, they are therefore expensive and it is important to reduce the amounts used in order to render the PEM water electrolysis technology viable.
At the cathode, platinum is generally used for the production of hydrogen. In order to limit the amounts of platinum of the electrolyzer cores, carbon supports (powders, sheets, etc.) are used. These supports are very good electron conductors and are stable under the cathode conditions.
At the anode, as mentioned previously, the anode materials must withstand high potentials (>1.5 V/SHE).
Thus, the use of carbon-based supports cannot be envisaged since these oxidize rapidly (formation of CO2). The oxides of noble metals (oxides of iridium IrO2, bi-metal oxides, etc.) are predominantly used in the anodes of PEM electrolyzers since they are good electron conductors and have advantageous electrocatalytic properties with respect to the electro-oxidation of water but also a good chemical stability with respect to the high operating potentials (2 to 3 V).
Generally, these materials are therefore used as electrocatalysts and ensure the good electron conductivity of the layer (no catalyst support). Nevertheless, as they are generally dense, in order to obtain sufficient electroactive surfaces, the anode loadings of electrocatalytic materials are often very high, of the order of 2-3 mg·cm−2. These oxides of noble and precious metals are expensive and it is essential to reduce these loadings without however affecting the electrocatalytic activity and the electron conduction of the electrode. It should be noted that below 0.5 mg·cm−2 of anode catalysts, the electron percolation of the layer becomes difficult (too little material). This is why higher loadings are often encountered.
Much research has been carried out in order to reduce the amount of noble metals used at the anode. Mention may in particular be made of:
the production of bi- or tri-metal materials composed of elements that are less noble but stable combined with iridium or with ruthenium (Xu Wu, Jyoti Tayal, Suddhasatwa Basu, Keith Scott, Nano-crystalline RuxSn1-xO2 powder catalysts for oxygen evolution reaction in proton exchange membrane water electrolysers, International Journal of Hydrogen Energy, 36, no. 22, 2011, 14796-14804);
the search for catalyst support (similar to the carbon used at the cathode): this support must be chemically stable and a good electron conductor. Among the supports conventionally encountered, mention may in particular be made of:                titanium suboxide (TiO2-x), in particular described in U.S. Pat. No. 5,173,215, “Conductive titanium suboxide particulates”. Its stability has not been studied much but it reoxidizes rapidly to non-conductive TiO2 at the operating potentials used in PEM water electrolysis;        ATO: antimony-doped tin oxide (Marshall, A. T., Haverkamp, R. G., Electrocatalytic activity of IrO2—RuO2 supported on Sb-doped SnO2 nanoparticles, 2010, Electrochimica Acta, 55 (6), pp. 1978-1984).        
It should be noted that since such supports are not very conductive, it is necessary to use high loadings of noble metals so as to at least 60% cover the support particles.
The encouraging results proposed by these authors are only presented for anode catalyst loadings of greater than or equal to 1 mg·cm−2. Yet, for loadings of greater than 0.5 mg·cm−2, the electron conduction in the anode layer is ensured and the performance changes very little with the loading; the catalyst support is not useful in those cases.
It has also been proposed, in patent US 2011/0207602 A1 (Nanometer powder catalyst and its preparation method), to use titanium oxide nanoparticles as catalyst support for applications in PEM water electrolysis. The very good performances obtained with this non-conductive catalyst support are only explained by the high loading used (1.24 mg·cm−2 of IrO2), the non-conductive TiO2 particles are covered by noble metal particles.
For this loading, the support has no effect since the active layer is continuous. Thus, from the perspective of a low loading (typically less than 0.5 mg/cm2), the use of these non-conductive particles is proscribed since they must ensure the electrical continuity of the active layer.
It has also been described, in the publication J. Polonsky, I. M. Petrushina, E. Christensen, K. Bouzek, C. B. Prag, J. E. T. Andersen, N. J. Bjerrum, Tantalum carbide as a novel support material for anode electrocatalysts in polymer electrolyte membrane water electrolysers, International Journal of Hydrogen Energy, 37, no. 3, 2012, 2173-2181, to use TiC as catalyst support for IrO2. However, the Applicant has demonstrated that this type of material (TiC, TiN, etc.) is not stable under the conditions for the electrolysis of water; moreover, they do not present any durability test.
Thus, many studies have focused on the nature of the electrocatalytic species in terms of performance.
Within this context, the Applicant is also interested in the unit formed by the catalyst-loaded active layer in contact with the current collector present in the hydrogen production device, in order to improve the electrical conduction thereof.
Indeed, the anode current collector carries out the dual role of bringing water (reactant) into contact with the active layer and of discharging the gas produced (O2).
It has already been proposed to manufacture the electrode directly on the porous titanium material (Wu, X., Tayal, J., Basu, S. & Scott, K., Nano-crystalline RuxSn1-xO2 powder catalysts for oxygen evolution reaction in proton exchange membrane water electrolysers, International Journal of Hydrogen Energy, 36, 14796-14804 (2011)), the catalyst/current collector contact resistances having to be improved thereby, however due to a poor electrical contact between the current collector and the active layer with the solid electrolyte, the performance is not satisfactory.
A wealth of literature exists in the field of structuring electrodes or current collectors, this being for various fields of application. Authors have proposed in particular, in patent application US 2013/0128412, an optimized, fluted collector shape, covered by a conductive layer, which makes it possible to obtain good electronic contacts with the active layer. A particular architecture has also been proposed in patent application US 2013/0101896, for improving the electrical contacts of the external battery connectors. Conductive studs have also been produced by anodic oxidation on a current collector substrate in patent application US 2013/0101902.
Within the field of the present invention, solutions have also been proposed for improving the electrical contacts between the current collector grids and the electrodes.
For applications in lithium-ion batteries, carbon studs have been manufactured on current collector grids in order to obtain a greater contact area with the electrode (described in patent application WO 2013/61889).
Patent application US 2010/0086849 proposes to incorporate a first portion of the electrode in the current collector by acting on the choice of two particle sizes so as to obtain an improved electrical contact in the electrode/current collector unit.
These two solutions require several manufacturing steps, the first for structuring the current collector, the second for depositing the electrode. Furthermore, the solution proposed in application US 2010/0086849 is not compatible with the field of the invention; it does not make it possible to create a collector with a sufficient porosity to produce an effective current collector.